1. Field of the Invention
The present invention relates to the geometry of artificial teeth for front-and molar-tooth portions, which are formed of a synthetic resin or a composite material of synthetic resins and ceramics, and have an angle of retention formed to their basal planes.
2. Description of the Prior Art
Heretofore, a number of geometries have been imparted to the basal planes of artificial teeth so as to augment their bonding strength with respect to plate resins. Until now undercuts or retaining pins have been used for ceramic artificial teeth in particular, because they should always be retained by means of a mechanical retaining force due to the fact that they show no scientific bond strength with respect to plate resins whatsoever. As regards artificial teeth formed of a synthetic resin or a composite material of synthetic resins and ceramics, however, some scientific bonding strength may be expected with respect to plate resins, since they are formed of the same polymethyl methacrylate or its derivative as the plate resins. For that reason, the geometries of the basal planes of artificial teeth have not received attention at all, or even if some consideration has been given thereto, such geometries have had little effect in increasing their bonding strength with respect to plate resins. Referring to the geometries of the basal planes to which a significant attempt has been made in the art to increase the bonding strength between artificial teeth and plate resins, there have been only two methods; one for increasing the surface area of the basal planes and the other for adding a simple retaining hole thereto.
Thus, only three geometries, i.e., (1) a flat plane, (2) a plane curved to increase its surface area and (3) a plane having a simple retaining hole in it, have been available for the basal planes of artificial teeth.
The bonding strength between artificial teeth and plate resins plays an important role in the following five possible cases:
(1) Preparation of a denture (e.g., gypsum-indexing),
(2) Mastication with a denture put in the mouth (lateral, anterior/posterior and vertical movements of the jaws),
(3) Repair of a denture (especially, polishing for the occlusal equilibration of artificial teeth),
(4) Mishandling of a denture in use (dropping or treading), and
(5) Implantation of artificial teeth in a temporary fitting wax, when carrying out their try-in-the-mouth operation before preparing a denture.
Of these, cases (1) to (4) correlate to a bonding strength with respect to plate resins after polymerization, and case (5) relates the ease with which artificial teeth are implanted in a temporary fitting wax (for a instance, G-C Utility Wax) for a try-in-the-mouth operation and involves danger resulting from the disengagement of artificial teeth in the mouth.
The basal planes of artificial teeth so far available assume the following three possible geometries:
(1) a flat plane,
(2) a plane curved to increase its surface area, and
(3) a plane having a simple retaining hole in it.
In the five cases as mentioned above, these geometries are all insufficient in the bonding strength between artificial teeth and plate resins for the reasons which will be summarized just below.
(1) when preparing a denture, artificial teeth disengage from a plate resin. If the basal plane of an artificial tooth is flat, then not only does it have a limited surface area, but it is also only resistive to a simple tensile force and is ineffective with respect to a shearing force. If the basal plane of an artificial tooth is therein provided with a simple retaining hole, then a portion of the hole receiving a plate resin becomes a notch on which stress concentrates, thus leading to breaking of the plate resin. If the basal plane of an artificial tooth formed of a synthetic resin or a composite material of synthetic resin and ceramics is curved, then any artificial toothretaining effect is not expected, however much that plane is curved so as to increase its surface area due to a crosslinking agent being used for the most part therein.
(2) Since complicated movements are repeated by mastication in the mouth, stress is applied on the interface of a denture and a plate resin by lateral, anterior/posterior and vertical movements of the jaws, although gradually, with the result being that the denture is very much fatigued. The flat, curved and holed (for simple retaining) geometries of the basal planes so far available are not at all effective for such three-dimensional application of loads.
(3) After a denture has been put in the mouth and used over an extended period, it must necessarily be repaired. That is, artificial teeth should often be either polished with a carbon random point or anything similar or automatically milled-in with G-C lapping paste on an articulator. In such cases, interfacial breaking of the artificial teeth and a plate resin, due to fine vibrations of the point, is caused and proceeds so rapidly that the flat, curved and holed (for simple retaining) geometries of the basal planes often result in disengagement of the artificial teeth from the plate.
(4) The gravest problem with the use of dentures is inadvertant mishandling by patients themselves. For instance, the disengagement of artificial teeth from the associated plate is most frequently caused, when the dentures are dropped or trod upon with a large force. In such a case, the dentures receive an impact force, and so should possess bond strength toughness. However, the flat, curved and holed (for simple retaining) basal planes so far available cannot stand up to such an impact force.
(5) Before preparing a denture, the form and color tone of artificial teeth are selected. The thus selected suitable artificial teeth should then be implanted in a temporal fitting wax (for instance, G-C Utility Wax), which is to be tried in the mouth. However, artificial teeth which have their basal planes flattened, curved or holed for simple retaining cannot be temporally fitted to the wax in a satisfactory manner. In addition, such artificial teeth are very dangerous due to the fear that during a tri-in-the-mouth operation, they may disengage from the wax and block the throat or the bronchus, ending in suffocation.